1. Field of the Invention
The present invention relates to a method of continuous manufacturing of polymers or copolymers of trioxane. More particularly, it pertains to an improvement in the method of continuously manufacturing fine granular polymers from trioxane or by making copolymerization of trioxane as their main raw material with copolymerizable comonomers, using a polymerization reaction apparatus of the so-called continuous stirrer-mixer type having two parallel shafts which turn against each other, a large number of paddles mounted on each shaft and a barrel placed in proximity to the peripheries of said paddles.
2. Description of the Prior Art
Generally, two phenomena occur when polymerizable liquid monomers are polymerized in the presence of some appropriate polymerization catalyzer. In one, since the polymers formed are soluble in monomers, they form a viscous liquid, producing a highly viscous substance with proceeding polymerization. In the other, since the polymers are insoluble in the monomers, they undergo phase changes with proceeding polymerization from liquid, through the slurry state, to a perfect solid state of polymers which are deposited. This invention relates to the latter. The manufacture of polyacetal resins is a typical example of the continuous manufacture of solid polymers through such a polymerization reaction in the latter case.
The method of manufacturing polyacetal resins through homopolymerization of trioxane with cyclic ethers, such as ethylene oxide, etc., or cyclic formals, etc., using cationic active polymerization catalyzers such as boron trifluoride, phosphorus pentafluoride, tin tetrachloride and perchloric acid or their salts or complex salts, etc., is already well-known and is industrially practiced. In this polymerization or copolymerization reaction, the liquid monomers undergo so-called phase changes with proceeding polymerization from the liquid state of monomers through a slurry state for a short period to the solid state of polymers. Moreover, in the so-called block polymerization process in which almost no diluent is present, the reaction velocity is very high and, therefore, the phase changes are sudden and drastic, permitting no easy control. For example, if this polymerization or copolymerization reaction is performed in a stationary state, large lumpy, tough products will be obtained in a short period of time or almost instantaneously, resulting in great difficulty in their handling in the later grinding, cleaning and refining steps and the internal accumulation of polymerization heat makes temperature control almost impossible. This causes deterioration of the quality of the polymers and reduction in the polymerization conversion rate. At this juncture, numerous inventions have been proposed as methods for producing at high efficiency, fine granular polymer products which are relatively stabilized in quality by preventing the production of large block polymerization products, taking into account special aspects of the reaction. A basic idea common to those proposals involves the utilization of extruder-type polymerization reactors having a construction of two parallel shafts for stirring.
The idea of utilizing extruder-type reactors having a construction of two parallel shafts for the manufacture of polyacetal resins, were inspired by inventions based on use of extruders of two shaft screw-type of Patent Laid-Open Nos. 629/'72 and 42145/'72 and subsequently, another invention involving the utilization of a two shaft mixer composed of a combination of screws and elliptical plate shape paddles appeared in Patent Laid-Open No. 84890/'76. Numerous ingenious modifications and improvements have been made in the paddle shape, as shown by the successive proposals disclosed in Patent Laid-Open Nos. 86794/'78, 38313/'81 or 32619-21/'83. There is also available a type of reactor in which two parallel shafts turn in the same direction and another type of reactor in which they turn against each other (different direction turning). Both of these exhibit similar functions, but the former is characterized by an excellent self-cleaning property, while with the latter, an inventive proposal has been made, as in Patent Laid-Open No. 40520/'82, that shearing forces are developed in the desired altered directions corresponding to the phase changes. At present, most industrial production of polyacetal resin is carried out by manufacturing methods based on the above inventions.
Recently, the demand for polyacetal resin has increased and the quality of resin, especially where stability under heating is concerned, must be high. For these reasons, the present state of manufacturing is not fully satisfactory. Thus there is a demand to upgrade the polymer yield or polymerization conversion rate per unit apparatus and further, amelioration and improvement through increased efficiency of stabilization treatment. In the manufacture of polyacetal resin is made, using parallel two shaft rotary stirring type reactors based on the aforementioned proposals by applying various ingenious design to the shape of paddles mounted on the rotational shafts and their arrangements, relatively fine granular polymers will be obtained at high conversion rates in small scale apparatuses such as those for use in laboratories. However, with increasing sizes of the apparatus, satisfactory results have not always been achieved. As the yield of polymers with large grain sizes (e.g., coarse granules having sizes as large as or larger than a little finger), rather than fine granules, grows and the thickness of the polymer scale layer which is deposited on the internal wall surface of the apparatus increases, a situation develops in which a reduction in the polymerization conversion rate and the degradation of polymer quality due to decline in heat transfer efficiency are unavoidable to some extent.
When the bulk (co)polymerization of trioxane is performed at a very high reaction velocity and the formed polymers are insoluble in the material monomers, during the proceeding polymerization reaction, an abrupt change from slurry to solid state takes place, with the result being that the formed polymer particles tend to form large blocks followed by an increase in the temperature of the polymer due to internal storage of reaction heat, causing decomposition of polymers and a reduction in quality. If the polymers can be finely granulated at the polymerization step, the decomposition may be inhibited with the storage of heat of the polymerization reaction being averted. As a result, the polymerization yield is upgraded, the fall in the degree of polymerization is slight and the production rate of unstable part in copolymers is small. For this reason, after-steps are greatly retrenched and the successful manufacture of trioxane (co)polymers with high quality is anticipated. However, the aforementioned inventions still involve various short comings.
In reactors with two parallel shafts which are turned in the same direction, no nipping-in between paddles takes place thereby resulting in insufficient effect in fine granulation and inadequate mixing of reaction reagents, and besides, need for use of large power.
In the reactors using lens-shaped paddles or pseudo-triangular paddles which have heretofore been in use, with their shafts turnable in different directions, the aforementioned difficulties may be eliminated. However, it is instantaneous only when the distance between paddles is at a minimum and in other times, they turn with large clearances, so that there exists great opportunity for polymers making ling into large blocks.
Furthermore, there has been a problem in that scale tended to be deposited on the inside wall surface of the barrel. For elimination of such a problem, paddles provided at their tips with sharp scrapers and which turn in the same direction were proposed (Patent Laid-Open Nos. 383131/'81 and 3262/'83). In these reactor types scraping beyond the clearance between the paddle tips and the inside wall was impossible because in large capacity reaction apparatuses, it was impossible to reduce this clearance to a predetermined value by way of machining or because the paddles and the inside wall surface and the two mated paddles come in contact with each other. Thus, this type of paddle offered only an unsatisfactory means of scale removal, but no other effective measure was available.